The present invention relates to an exhaust gas purification device of an internal combustion engine.
In a diesel engine, at the time of low speed, low load operation of the engine, in particular at the time of warmup operation of the engine, the temperature inside the combustion chamber becomes lower and as a result a large amount of unburned hydrocarbons is generated. Therefore, known in the art has been a diesel engine having an exhaust control valve arranged in an engine exhaust passage, closing the exhaust control valve and greatly increasing the amount of fuel injection at the time of engine low speed, low load operation so as to raise the temperature in the combustion chamber and cause the injected fuel to completely burn in the combustion chamber and thereby keep down the amount of generation of unburned hydrocarbons (see Japanese Unexamined Patent Publication (Kokai) No. 49-801414).
Further, when arranging an exhaust purification catalyst in an engine exhaust passage, if the temperature of the catalyst does not become sufficiently high, a good exhaust purification action is not obtained by the catalyst. Therefore, known in the art is an internal combustion engine which injects auxiliary fuel during the expansion stroke in addition to the injection of the main fuel for generating the engine output and causes the auxiliary fuel to burn so as to cause the temperature of the exhaust gas to rise and thereby cause the temperature. of the catalyst to rise (see Japanese Unexamined Patent Publication (Kokai) No. 8-303290 and Japanese Unexamined Patent Publication (Kokai) No. 10-212995).
Further, known in the art is a catalyst able to absorb unburned hydrocarbons. This catalyst has the property that the higher the surrounding pressure, the greater the amount of absorption of unburned hydrocarbons and that when the pressure of the surroundings becomes lower, the absorbed unburned hydrocarbons are released. Therefore, known in the art is an internal combustion engine which utilizes this property to reduce the NOx by the unburned hydrocarbons released from the catalyst by arranging this catalyst in an engine exhaust passage, arranging an exhaust control valve inside the engine exhaust passage downstream of the catalyst, injecting a small amount of auxiliary fuel during an expansion stroke or exhaust stroke in addition to main fuel for generating engine output at the time of engine low speed, low load operation when the amount of generation of NOx is small, exhausting a large amount of unburned hydrocarbons from the combustion chamber, closing the exhaust control valve to a relatively small opening degree at this time so that the drop in engine output falls within the allowable range so as to raise the pressure inside the exhaust passage and cause a large amount of unburned hydrocarbons exhausted from the combustion chamber to be absorbed in the catalyst, fully opening the exhaust control valve to cause the pressure in the exhaust passage to fall at the time of engine high speed or high load operation when the amount of generation of NOx is large, and reducing the NOx by the unburned hydrocarbons released from the catalyst at this time (see Japanese Unexamined Patent Publication (Kokai) No. 10-238336).
Further, current diesel engines of course and also spark ignition type internal combustion engines have the major problem of how to reduce the amount of unburned hydrocarbons generated at the time of engine low load operation, in particular at the time of warmup operation of the engine. Therefore, the present inventors engaged in experiments and research to solve this problem and as a result found that to greatly reduce the amount of unburned hydrocarbons exhausted into the atmosphere at the time of warmup operation of an engine etc., it is necessary to reduce the amount of unburned hydrocarbons generated in the combustion chamber and simultaneously to increase the amount of reduction of unburned hydrocarbons in the exhaust passage.
Specifically speaking, they learned that if auxiliary fuel is additionally injected into the combustion chamber during the expansion stroke or exhaust stroke and that auxiliary fuel burned and if an exhaust control valve is provided in the engine exhaust passage a considerable distance away from the output of the engine exhaust port and the exhaust control valve is made to substantially completely close, the synergistic effect of the combustion of the auxiliary fuel and the exhaust throttling action due to the exhaust control valve causes the amount of generation of the unburned hydrocarbons in the combustion chamber to fall and the amount of unburned hydrocarbons in the exhaust passage to rise and thereby enables the amount of unburned hydrocarbons exhausted into the atmosphere to be greatly reduced.
Explaining this a bit more specifically, when auxiliary fuel is injected, not only is the auxiliary fuel itself burned, but also the unburned hydrocarbons left over after burning the main fuel is burned in the combustion chamber. Therefore, not only is the amount of unburned hydrocarbons generated in the combustion chamber greatly reduced, but also the unburned hydrocarbons remaining after burning the main fuel and the auxiliary fuel are burned, so the temperature of the burned gas becomes considerably high.
On the other hand, if the exhaust control valve is substantially completely closed, the pressure in the exhaust passage from the exhaust port of the engine to the exhaust control valve, that is, the back pressure, becomes considerably high. A high back pressure means that the temperature of the exhaust gas exhausted from the combustion chamber does not fall that much. Therefore, the temperature of the exhaust gas in the export port becomes considerably high. On the other hand, a high back pressure means that the flow rate of the exhaust gas exhausted in the export port is slow. Therefore, the exhaust gas remains in a high temperature state in the exhaust passage upstream of the exhaust control valve over a long period of time. During that time, the unburned hydrocarbons contained in the exhaust. gas are oxidized and therefore the amount of unburned hydrocarbons exhausted into the atmosphere is greatly reduced.
In this case, if auxiliary fuel were not injected, the unburned hydrocarbons left over after burning the main fuel remain as they are, so a large amount of unburned hydrocarbons is generated in the combustion chamber. Further, if auxiliary fuel were not injected, the temperature of the burned gas in the combustion chamber would not become that high, so even if substantially fully closing the exhaust control valve at this time, a sufficient action in purifying the unburned hydrocarbons in the exhaust passage upstream of the exhaust control valve could not be expected. Therefore, at this time, a large amount of unburned hydrocarbons would be exhausted into the atmosphere.
On the other hand, even if not throttling the exhaust by the exhaust control valve, if injecting auxiliary fuel, the amount of unburned hydrocarbons generated in the combustion chamber is reduced and the temperature of the burned gas in the combustion chamber becomes high. When not throttling the exhaust by the exhaust control valve, however, the pressure of the exhaust gas immediately falls after the exhaust gas is exhausted from the combustion chamber and therefore the temperature of the exhaust gas immediately falls. Therefore, in this case, almost no action of oxidation of the unburned hydrocarbons in the exhaust passage can be expected and therefore a large amount of unburned hydrocarbons is exhausted into the atmosphere at this time as well.
That is, to greatly reduce the amount of unburned hydrocarbons exhausted into the atmosphere, it becomes necessary to inject auxiliary fuel and simultaneously substantially fully close the exhaust control valve.
In the diesel engine described in the above Japanese Unexamined Patent Publication (Kokai) No. 49-80414, no auxiliary fuel is injected and the amount of main fuel injected is greatly increased, so the temperature of the exhaust gas rises, but an extremely large amount of unburned hydrocarbons is generated in the combustion chamber. If an extremely large amount of hydrocarbons is generated in the combustion chamber, even if there is an oxidation action of the unburned hydrocarbons in for example the exhaust passage, only part of the unburned hydrocarbons will be oxidized, so a large amount of unburned hydrocarbons will be exhausted into the atmosphere.
On the other hand, in the internal combustion engine described in the above-mentioned Japanese Unexamined Patent Publication (Kokai) No. 8-303290 or Japanese Unexamined Patent Publication (Kokai) No. 10-212995, since there is no exhaust throttling action by the exhaust control valve, almost no action in oxidizing the unburned hydrocarbons in the exhaust passage can be expected. Therefore, even in this internal combustion engine, a large amount of unburned hydrocarbons is exhausted into the atmosphere.
Further, in the internal combustion engine described in the above-mentioned Japanese Unexamined Patent Publication (Kokai) No. 10-238336, the exhaust control valve is closed to a relatively small opening degree so that the drop in output of the engine falls within an allowable range and therefore in this internal combustion engine, the amount of main fuel injected is maintained at an identical amount of injection when the exhaust control valve is fully opened and when it is closed. With an amount of closure of the exhaust control valve of an extent where the drop in engine output falls within an allowable range, however, the back pressure does not become that high.
Further, in this internal combustion engine, to generate the unburned hydrocarbons to be absorbed in the catalyst, a small amount of auxiliary fuel is injected into the expansion stroke or exhaust stroke. In this case, if the auxiliary fuel can be burned well, no unburned hydrocarbons are generated any longer, so in this internal combustion engine, the injection of the auxiliary fuel is controlled so that the auxiliary fuel is not burned well. Therefore, in this internal combustion engine, it is believed that a small amount of auxiliary fuel does not contribute that much to the rise in temperature of the burned gas.
In this way, in this internal combustion engine, it is believed that a large amount of unburned hydrocarbons is generated in the combustion chamber and further the back pressure does not become that high and the temperature of the unburned gas does not rise that much, so the unburned hydrocarbons are not oxidized that much even in the exhaust passage. In this internal combustion engine, the objective is to cause as large an amount of unburned hydrocarbons to be absorbed in the catalyst. Therefore, thinking in this way can be said to be rational.
An object of the present invention is to provide an exhaust gas purification device of an internal combustion engine able to ensure stable operation of the engine and greatly reduce the amount of unburned hydrocarbons exhausted into the atmosphere.
According to the present invention, there is provided an exhaust gas purification device of an internal combustion engine wherein an exhaust control valve is arranged a predetermined distance away from an outlet of an engine exhaust port inside an exhaust passage connected to the outlet of the exhaust port; when it is judged that the amount of unburned hydrocarbons exhausted into the atmosphere is to be reduced, the exhaust control valve is substantially fully closed and, in addition to burning the main fuel injected into the combustion chamber under excess air to generate engine output, auxiliary fuel is additionally injected into the combustion chamber at a predetermined timing in the expansion stroke or exhaust stroke where the auxiliary fuel can be burned so that the amount of unburned hydrocarbons produced in the combustion chamber is reduced and the oxidizing reaction of hydrocarbons in the exhaust port and the exhaust passage upstream of the exhaust control valve is promoted; and when the exhaust control valve is substantially fully closed, the amount of injection of main fuel is increased compared with the case where the exhaust control valve is fully opened under the same engine operating conditions so as to approach the torque generated by the engine when the exhaust control valve is fully opened under the same engine operating conditions.